1. Field of the Invention
This invention relates to a magnetically controlled transformer including magnetic cores and windings by which AC output power can be accurately controlled by means of a source of DC control current and a DC control winding.
2. Background Art
Examples of previously used magnetic circuits for controlling AC output power are described while referring to the drawings, where FIG. 1 shows a magnetic circuit 1 including a single core saturable reactor 2. Saturable reactor 2 has a winding 4 which is wound around a saturable magnetic core, such that the inductance of winding 4 will vary with the flux density of the magnetic core material. The flux density of the magnetic core material varies over time with an applied AC voltage from a suitable source 6 thereof typically having an applied frequency which ranges from 10 Hz to 10 mHz. Saturable reactor 2 also includes a second winding 8 which is magnetically coupled to the core wound winding 4. Winding 8 usually has more turns than winding 4 so that a voltage or current gain will be produced across a load resistor 10. The second winding 8 is driven by a series connected DC current source 12. In operation, as the DC current from source 12 increases, the impedance or reactance of winding 4 decreases, while the output voltage across load resistor 10 increases. However, the AC voltage from source 6 which appears across winding 4, minus the voltage across load resistor 10, is also applied directly to the coupled winding 8. Thus, the magnetic circuit 1 of FIG. 1 is difficult to control, because the AC voltage which is fed back across winding 8 interferes with the current from source 12. Moreover, the circuit 1 may be unsafe, depending upon the turns ratio of the windings 4 and 8 and the corresponding magnitude of the fed back voltage. That is, if the number of turns on winding 4 is small compared to the number of turns on winding 8 and the voltage from AC source 6 is high, then the output voltage at load 10 is very high and possibly hazardous.
Another magnetic circuit shown in FIG. 1a included the single core saturable reactor of FIG. 1 with an additional winding 15 (sometimes referred to as a flying choke). That is, the additional winding 15, which was not magnetically coupled to either one of the saturable core windings (i.e. designated 4 and 8 in FIG. 1), is connected in series with winding 8 in the DC current path. This extra winding 15 advantageously absorbs some of the fed back AC voltage without affecting output power or control. However, the cost, size and weight of this magnetic circuit was increased because of the inclusion of the additional winding 15. Moreover, and inasmuch as the additional winding 15 had to sustain the fed back voltage, said winding has been known to break down or arc.
A third magnetic circuit 16, including a 2-core saturable reactor 18, is illustrated in FIG. 2 of the drawings. The saturable reactor 18 has two series connected windings 20 and 22 which are wound around respective balanced (i.e. equal inductance) magnetic cores and a third winding 24 which is magnetically coupled to each of the windings 20 and 22 to better absorb the fed back AC voltage. A DC current source 26 is connected in a DC current path to drive the common winding 24. The output voltage across the load resistor 28 of circuit 16 is proportional to the DC current from source 26. Thus, if no DC current is applied to common winding 24, there will be no fed back AC voltage to winding 24. That is, winding 24 cancels the fundamental frequency of AC source 30 resulting in no AC fed back voltage when the DC control current is zero. However, and as a disadvantage, when the common winding 24 is driven by a DC current, the second harmonic (as opposed to the fundamental) of the AC input voltage from source 30 appears across winding 24. The AC output voltage is controlled in a similar fashion to the circuit of FIG. 1, except that two series connected windings 20 and 22 are used which increases the size, weight, and cost of the saturable reactor 18.
A fourth known magnetic circuit represented in FIG. 2a included the 2-core saturable reactor of FIG. 2 with the inclusion of a vacuum tube 31 and a shunt connected capacitor 33. This modified circuit commonly used as AC input voltage operating at approximately 400 Hz and was particularly applicable for aircraft (e.g. for heaters, motors, and regulators). The vacuum tube 31 was added to more reliably control the DC current through the common, magnetically coupled winding (i.e. designed 24 in FIG. 2), while the capacitor 33 protected the vacuum tube 31 from the second harmonic of the fed back voltage. Hence, the gain of this circuit could be maximized to form a power amplifier. However, the added vacuum tube 31 consumed space, was sometimes unreliable and generated heat.
Therefore, it was desirable to eliminate the vacuum tube of FIG. 2a but still retain the high gain that was available by means of the aforementioned power amplifier. The foregoing was accomplished by the magnetic circuit 34 of FIG. 3 which included a pair of rectifiers 36 and 38. Each rectifier is shown connected in series with a respective winding 40 and 42 (sometimes referred to as gate windings) that is wound around a core formed from a magnetic material characterized by high permeability. In this manner, the rectifiers 36 and 38 would fire sequentially with the source 44 of AC input voltage, such that the circuit 34 was often referred to a gated magnetic amplifier. The circuit 34 also included a common control winding 46 which is magnetically coupled to each of the windings 40 and 42. A DC current source 48 is connected in a DC current path to drive the common winding 46. In operation, a high output voltage initially appears across the load resistor 50, and a small DC current is needed from source 48 to drive common inductor 46 and thereby control such output voltage. This provides high gain without the vacuum tube of FIG. 2a. More particularly, with no DC control current being applied from source 48, the full input voltage is reflected at the load resistor 50, such that the gated magnetic amplifier of FIG. 3 has been found unsuitable and even hazardous for many applications as a consequence of its normally on state.
With the advent of transistors, a center gated magnetic amplifier became available to produce either pulsed AC or DC output voltage. The corresponding circuit 52 illustrated in FIG. 4 of the drawings included four magnetic cores, eight gate windings, two control windings, and a reset resistor 53. The circuit 52 advantageously avoided the normally on state of the circuit of FIG. 3. However, the problems with center gated magnetic amplifier 52 were its large size and the power that had to be dissipated in the reset resistor 53 to reset the magnetic cores for consecutive firing. Consequently, the efficiency of this circuit was reduced by at least 50 percent, since half of the input power from the AC voltage source is dissipated in reset resistor 53.
FIG. 5 of the drawings shows a relatively recent circuit 54 which eliminated the multiple cores and reset resistor of the aforementioned center gated magnetic amplifier of FIG. 4. The foregoing was accomplished by means of using thyristors or silicon controlled rectifiers (as shown), triacs, etc., instead of magnetic cores. A circuit of this nature was desirable because of its efficiency and relatively small size, inasmuch as there was no longer a need to dissipate power in a reset resistor.
Examples of these and other prior art magnetic circuits are available by referring to one or more of the following U.S. Pat. Nos.:
1,815,516: July 21, 1931 PA1 1,910,381: May 23, 1933 PA1 2,498,475: Feb. 21, 1950 PA1 2,870,397: Jan. 20, 1959 PA1 3,087,108: Apr. 23, 1963 PA1 3,123,764: Mar. 3, 1964